Fix Flatpak missing locale support (#12714)

Add the localization/ directory to the Flatpak source list so
run_gettext.sh can compile .po files into .mo translations.

Replace LC_ALL=C.UTF-8 with LC_NUMERIC=C in the entrypoint script
to preserve the user's language settings while still preventing
decimal-separator parsing issues.

scripts/flatpak/io.github.orcaslicer.OrcaSlicer.yml

@@ -347,6 +347,9 @@ modules:
       - type: dir
         path: ../../src
         dest: src
+      - type: dir
+        path: ../../localization
+        dest: localization
 
       - type: file
         path: ../../CMakeLists.txt